Preparation of oxidation products of cyclohexane



nited States PREPARATIGN OF GXIDATEUN PRODUCTS F CYCLOHEXANE Harry LouisCates, Jr., .lohn Ulivcr Punderson, and Robert William Whcatcroft,Wilmington, Bet, and Alvin Barber Stiles, Charleston, W. Va., assignorsto E. I. du Pont de Nemours and Company Wilniin ton Del. a corporationof Delaware g No Drawing. Application July 27, 1954 Serial No. 446,176

6 Claims. (Cl. 260-586) This invention relates to an improved processfor preparing adipic acid and precursors thereof.

The art of oxidizing organic compounds in the liquid phase developedrapidly in recent years,

of oxidizing cyclohexane in the liquid phase hexanol and cyclohexanone.In the Loder process the preferred catalysts included cobaltnaphthenate, and the preferred oxidation initiators included suchmaterials as ket-ones, aldehydes, peroxides,

cyclohexane oxidized was less than When the quantity of cyclohexaneoxidized was from about 5% to about 12%, the yield ofcyclohexanol-cyclohexanone was about 65% to about 85%. It has morerecently been reported '(Farkas et al., U. S. 2,410,- 642) that underconditions similar to those previously used, but in the absence ofLoders catalysts, quantitative yields of oxidation products 2,497,349)that the hydroperoxide which is formed under conditions similar to thosepreviously used for oxidation of cyclohexane can be converted tocyclohexanol by the action of reducing agents such as ferrous salts.

The thermal decomposition of pure or relatively concentrated cyclohexylhydroperoxide yields cyclohexanone as a main product, while the chemicalreduction of cyclohexyl hydroperoxide yields primarily cyclohexanol. Ifthe oxidized product is to be used for making adipic acid by further airoxidation, it is highly desirable to produce cyclohexanone, since theyield of adipic acid jhy air oxidation of cyclohexanone is much higherthan by over, it has also been disclosed (Farkas et al., U. S.

atent 235L496 Patented Sept. 9, 1958 sition of cyclohexyl hydroperoxideso as to produce a high proportion of ketone to alcohol. It is, however,highly desirable in that process to effect a reaction between cyclohexylhydroperoxide and cyclohexane, for such a reaction gives risestoichiometrically to two mols of cyclohexanol, instead of only one molof cyclohexanol or cyclohexanone otherwise obtainable from cyclohexylhydroperoxide. One of the advantages of the present invention is that itprovides a hydroperoxide decomposition step wherein, if desired,somewhat more than one mol of adipic acid precursors is formed from eachmol of cyclohexyl hydroperoxide which decomposes. It is to peroxides, inaddition to cyclohexyl hydroperoxide, are generally present at least tosome extent in the mixtures in question, and that these other peroxidesalso can undergo decomposition.

It is an object of this invention to provide an improved process for theoxidation of naphthenic hydrocarbons such as cyclohexane. Another objectof the invention is to expedite and control the decomposition of thenaphthenic hydroperoxide which is produced as one of the primaryoxidation products, in the oxidation of naphthenes so as to produce animproved yield of desirable dibasic acid precursors. A further object isto control this decomposition of peroxides formed by cyclohexaneoxidation without introducing any reducing agent or other reactant froman external source. Still another object is to control thisdecomposition of peroxides so as, if desired, to form cyclohexanoneWithout simultaneously forming cyclohexanol. Other objects of theinvention will appear hereinafter.

The present invention provides a process wherein naphthenichydroperoxides are produced by liquid phase oxidation of naphthenes witha gas containing molecular oxygen, following which these hydroperoxidesare destroyed in a separate step, without adding a reducing agent orother reactant prior to separation of the oxidized naphthenes(comprising monoketonaphthene and monohydroxynaphthene) from theoxidation mixture. The destruction of the hydroperoxide is achieved byincluding in the overall process, after from 1 to 12% or more of thenaphthene molecules have been oxidized, a controlled decomposition ofperoxides on a bed of solid catalyst in the absence of any reducingagent or oxygen. Thus the present invention includes the step ofcontrolling the thermal decomposition of peroxide, prior to distillationof the reaction products, by carrying out this decomposition in aseparate step using a bed of solid catalyst, rather than effecting theperoxide decomposition incidentally,

during distillation of the partial oxidation products, as in the priorart process.

The process of this invention is of considerable value in connectionwith processes wherein crude mixed oxidation products are converted bylater oxidation (e. g., with nitric acid) to dibasic acids, andespecially where the material subjected to nitric acid oxidation is avolatile fraction, free from relatively non-steam volatile products. Theeconomic advantages of the latter type of process for oxidation ofcyclohexane are explained in further detail in U. S. patent applicationof Goldbeck and Johnson, 8. N. 390,634, filed November 6, 1953, nowPatent No. 2,703,331. To obtain the maximum yield of steamvolatileadipic acid precursors, While producing the minimum amount of impurityto be removed in relatively expensive recrystallization processes, aperoxide decomposition step is especially helpful, and there areadvantages in performing such a step without introducing addedquantities of soluble catalyst or reducing agent.

In general, it may be desirable, although it is not absolutelynecessary, to employ an oxidation catalyst in the oxidation of thenaphthenic hydrocarbons hereinabove described. A suitable temperature is50 to 200 C., preferably 140 to 175 C. and at a pressure of 50 to 750lbs. per sq. in. If a catalyst is employed the quantity thereof shouldbe relatively small, e. g., an amount of cobalt containing orchromium-containing catalyst corresponding to l to 5000 parts permillion of Co or Cr may be present, but higher quantities generally arenot required.

Temperatures in the naphthene hydroperoxide decomposition step aresuitably to 300 C., preferably 50 to 175 C.

In a specific embodiment the invention can be practiced by oxidizingcyclohexane with an oxygen-containing gas, such as air, under thetemperature and pressure conditions defined hereinabove, continuing theoxidation until from 1 to 12% of the cyelohexane molecules have beenoxidized, whereby an oxidation product containing cyclohexanol.cyclohexanone and cyclohexyl hydroperoxide is obtained, and heating theresulting mixture, in the presence of a bed of solid peroxidedecomposition catalyst, in the absence of added reactant at atemperature within the range of 100 to 300 C. preferably 125 to 200 C.until disappearance of the said hydroperoxide takes place, and thereupondistilling the resulting mixture for recovery of unreacted cyclohexaneand also for recovery of cyclohexanol and cyclohexanone or mixturesthereof. If desired, the cyclohexanol and cyclohexanone need not beseparated from the relatively small amounts of other oxidation productswhich are present in the distillation residues. A convenient method forutilizing the oxidation products and hydroperoxide decompositionproducts thus obtained is to convert them to adipic acid by the nitricacid method disclosed in the Hamblet et al. patents U. S. 2,439,513,2,557,281 and 2,557,282.

.he form or shape of the reaction vessels employed in practicing theinvention, in both the oxidation andhydroperoxide decomposition steps,is of relatively minor consequence, suitable types being tubularconverters, upright towers, packed columns, vessels equipped withdevices for producing agitation or rapid flow, aerosol sprays. etc. Theprocess may be carried out in a continuous or batchwise manner. The samereaction vessel may be employed in both the oxidation and hydroperoxidedecomposition steps, but it is usually more convenient to use separatevessels for these distinctly different steps of the process.

In the continuous reaction systems, viscous or turbulent flow may beused. The vessels may be made of or lined with inert materials such asstainless steel, aluminum, tantalum, noble metals, ceramics, etc. Thevessels should of course be sufficiently strong to withstand theprevailing pressures, which in both the oxidation step and in thehydroperoxide decomposition step should be sufficiently high to maintainthe naphthene largely in the liquid phase, and from atmospheric pressureto about 1000 lbs. per sq. in. gauge or higher. The preferred pressurein the oxidation step is about 50 to 750 lbs. per sq. in. In theperoxide decomposition step the pressure is not particularly critical,although it is desirable to employ pressures high enough to permit thenaphthene to remain largely in the liquid phase at the prevailingtemperature.

At least a part of the unreacted naphthene may be recovered from theoxidizer efiiuent prior to performing the peroxide ecomposition step, ifdesired, but this is not a preferred practice. The hydroperoxide shouldnot be concentrated in the extent that only a relatively small amount ofdiluent naphthene remains, however, because lit concentrated naphthenehydroperoxide can undergo a spontaneous reaction which is almostexplosive in its violence.

In the oxidation step the gas containing molecular oxygen may be air,pure oxygen, oxygen-enriched air, air containing relatively inertgaseous diluents and the like.

In the oxidation step the reaction time, i. e. the time during which theoxygen-containing gas is in contact with the oxidizable naphthene,should preferably be controlled. that the conversion (i. e. thepercentage of molecules oxidized to all products) is within the range ofabout 1% to 12%. Usually the percentage of peroxide formed increasesvery rapidly with time until a maximum is reached, after which theperoxide content of the oxidation mixture decreases rapidly. For mosteffective results from the standpoint of the incremental increase inyield, it is desirable to remove the oxidation mixture from theoxidation zone, e. g. by physically transferring it to the hydroperoxidedecomposition vessel, when the proportion of hydroperoxide in theoxidized product is close to the maximum. This occurs when the totalconversion is quite low, a most preferred range of conversion being from3 to 9%. The mechanical'losscs are, of course, kept at the lowestpracticable minimum. Generally, the quantity of residual oxidationcatalyst dissolved in the hydrocarbon phase at the end of the oxidationstep is even smaller than was present initially in the oxidation step.

cyclohexyl hydroperoxide is itself convertible to adipic acid by nitricacid oxidation. In fact, the nitric acid oxidation of cyclohexylhydroperoxide yields adipic acid in high yields under the conditionsdisclosed in U. S. Patents 2,439,513 and 2,557,282 for oxidation ofprimary oxidation products of cyclohexane, the quantity of adipic acidproduced being nearly 1.0 mol per mol of hydroperoxide. Accordingly, inthe manufacture of adipic acid by the processes of the above-citedpatents the distillation residue obtained by recovery of cyclohexanefrom the total crude oxidation products when subjected to further nitricacid oxidation will produce, in addition to the adipic acid formed fromother components of the residue one mol of adipic acid per mol ofcyclohexyl hydroperoxide. If, however, a peroxide decomposition step isinterposed, and is carried out under such conditions of temperature,catalyst composition and concentration, pressure, etc. that the peroxidedecomposes in part by the following mechanism 051113 CgHnOOH 20011 011(oyclohexane) (cyclohexylhydroperoxide) (cyclohexanol) it is apparentthat during the said nitric acid oxidation step, every mol of cyclohexylhydroperoxide reacting in this manner will be converted, during thesubsequent nitric acid oxidation, to two mols of adipic acid, ratherthan one. The conversion to adipic acid is thus increased withoutsacrifice in yield.

One of the important facets of this invention is the discovery of theconditions required for converting the hydroperoxide (with cyclohexane)to more than one mol of adipic acid precursors, instead of only one. Incopending U. S. patent application of R. W. Wheatcroft and W. Warner S.N. 429,366, filed May 12, 1954, it is disclosed that cobalt catalysts,such as cobalt naphthenate, are effective for this bimolecularproduction of the precursor. To effect the bimolecular reaction, theamount of, this catalyst in many instances need not be any greater thanis present in the effluent from commercial cyclohexane oxidizers. Thesituation is, however, complicated somewhat by the further discoverythat cyclohexanone is also a catalyst for cyclohexyl hydroperoxidedisappearance in cyclohexane solutions at elevated temperatures. Acidsare also catalysts for this decomposition. Oxygen, on the other hand,has no such accelerating effect. The complete mathematical analysis ofthese variables is not essential to an understanding of the presentinvention, it

peroxide-containing fraction thereof, is conducted through a bed ofsolid catalyst, hereinafter described, at about 0, under sufiicientpressure to maintain a liquid phase, until the hydroperoxide content hasdrop- The cobalt concentration in the peroxide decomposition vessel, inthe process of this invention, is not enhanced by further addition ofcobalt naphthenate, as in the Punderson application, S. N. 279,239,filed March 28, 1952.

The solid of salts such as like.

While the decomposition can be made to occur thermally, without anyadded catalyst, this is a process which requires a sizeable hold-uptank, since the non-catalytic process is comparatively slow even atelevated temperatures and is present invention is that it speeds makingit almost instantaneous, thus eliminating from the process any need fora large hold-up tank; moreover, the invention accomplishes thisdesirable result without introducing into fire system any newcontaminants.

A series of experiments was performed in which numerous solid catalystswere compared to determine their effect upon decomposition of cyclohexylhydroperoxide in cyclohexane oxidizer efiluent. The efiduent used inthese tests was taken from the third oxidizer in a battery of threeair-oxidizers wherein cyclohexane was converted to primary oxidationproducts at 147 to 160 C. as described in U. S. 2,557,281. The contacttime in these tests was somewhat less than one minute, and thetemperature was maintained at 125 to 135 C.

Table I.Peroxide decomposition in crude cyclohexane oxidation productsof low peroxide content, on fixed bed catalysts (conversion tocyclohexanol-l-cyclohexanone, 5.8 to 6.6%)

Peroxide (09.10. as cyclohexyl up the decomposition,

Catalyst hydroperoxlde) percent conversion Control 0. 6 1 l 0. 2 0. 0. 23 Contr 0. 4 Ruthenium on activated a1 0.0 4 Control 0.3 Iron molybdateon activated alumina 0. 1

The foregoing table records data which show that each of the fixed bedcatalysts catalytically decomposed the' increase in total number of molsof adipic acid pre- Two of the most valuable catalysts in the practiceof M083, the

produced only crude product.

taining the granular catalyst. at a rate corresponding to the specifiedcontact time until a steady state was reached, whereupon the materialflowing into, and out of, the cartridge was sampled and analyzed.

It is significant to point out that an improvement of 0.4% incyclohexanol content is of major magnitude in this low conversionprocess, amounting to an increase of about 7% in the production capacityof the plant. In

composition step is especially The invention is further illustrated bymeans of the following example:

Example.Cyclohexane was oxidized with air at 169 C. under 160 lbs. persq. in. pressure with 1 p. p. m. cobalt (added as naphthenate) catalyst,until the peroxide content of the reaction mixture reached 1% by weight(peroxide calculated as cyclohexyl hydroperoxide). The total conversionat this point was 4%. The resulting mixture was conducted over apowdered reduced sintered cobalt oxide (C00) catalyst at C. at a contacttime of 15 minutes. The peroxide content of the mixture decreased as setforth in the following table,

decomposed peroxide molecules (calc. as cyclohexyl hydroperoxide). Thecyclohexanol/cyclohexanone mol ratio for the increased quantity ofcyclohexanoI-cyclohexanone is also given The experiment TablelI.-Dec0mp0sition of peroxides in crude cyclohexane oxidation productwith various fixed bed catalysts at 70 C. and 15 minute contact timePeroxide Catalyst Dccom- Percent K/A posed, KA 1 Ratio Percent M Cobaltoxide (000) -90 104 2. 9 Vanadium Oxide on Alumina 60-65 114 2. 6Molybdenum Sulfide 50-70 infinity Platinum on Carbon. 60-85 109 1.35Cobalt oxide on charcoal 75 103 1. 3

1 KA signifies cyclohexanone-cyclohexanol.

perature in the catalysts, a detectable quantity of peroxide remainsafter about 15 minutes time. At 100 C., the reaction takes less than aminute, and at higher temperatures the reaction is even more rapid.Generally it is not necessary or advantageous to use a temperature inexcess of the cyclohexane oxidation temperature, which preferably neednot exceed 175 C., although, when very low cyclohexane oxidationtemperatures are used, a higher temperoxide decomposition step isfeasible. Temperatures above the critical temperature of the oxidationproduct (mostly cyclohexane) are not employed.

One of the advantages of the process of this invention is that itpermits control over the peroxide content of the partial oxidationproducts prior to distillation of these products, while at the same timeincreasing the rate of production of adipic acid precursors, especiallythose which are recoverable in the.cyclohexanol-cyclohexanone steamdistillate, without the necessity for introducing any additionalcatalyst at the decomposition step. Where the decomposition step isomitted, the peroxide does not decompose appreciably during rapiddistillation of cyclohexane (unless the quantity of catalyst is large),and

hence the decomposition occurs either during distillation of the ketoneand alcohol, or

not at all. Where steam distillation is employed for recovery ofvolatile precursors of adipic acid, it is highly advantageous tointerpose the herein disclosed peroxide decomposition step prior to thesteam distillation step.

We claim:

1. In a process for liquid phase oxidation of cyclohexane with molecularoxygen, the steps which comprise effecting said oxidation at atemperature of from to 200 C. under a pressure of S0 to 750 lbs. per sq.in, heating at least a part of the resulting mixture at a temperature of30 to 300 C. in the presence of a granular peroxide-decompositioncatalyst of the class consisting of group VIII metals, molybdenumsulfide, iron molybdate-on-alumina, vanadium oxide-on-alumina, andcobalt oxide of the formula C00, until the cyclohexyl hydroperoxidecontent thereof has been reduced, and thereafter recoveringsteam-volatile partial oxidation products of cyclohexane from theresulting mixture by distillation.

2. Process of claim 1 wherein the oxidation of cyclohexane is carriedout at a temperature of to C.

3. Process of claim 1 wherein the decomposition of peroxide is effectedat 50 to 175 C.

4. Process of claim 3 wherein the said catalyst is CoO.

5. Process of claim 3 wherein the said catalyst is M08 and thedecomposition produces cyclohexanone without simultaneously producingcyclohexanol.

6. Process of claim 3 wherein the said catalyst is ruthenium-on-alumina.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PROCESS FOR LIQUID PHASE OXIDATION OF CYCLOHEXANE WITH MOLECULAROXYGEN, THE STEPS WHICH COMPRISE EFFECTING SAID OXIDATION AT ATEMPERATURE OF FROM 50* TO 200*C. UNDER A PRESSURE OF 50 TO 750 LBS. PERSQ. IN., HEATING AT LEAST A PART OF THE RESULTING MIXTURE AT ATEMPERATURE OF 30* TO 300*C. IN THE PRESENCE OF A GRANULARPEROXIDE-DECOMPOSITION CATALYST OF THE CLASS CONSISTING OF GROUP VIIIMETALS, MOLYBENUM SULFIDE, IRON MOLYBDATE-ON-ALUMINA, VANADIUMOXIDE-ON-ALUMA, AND COBALT OXIDE OF THE FORMULA COO, UNTIL THECYCLOHEXYL HYDROPEROXIDE CONTENT THEREOF HAS BEEN REDUCED, ANDTHEREAFTER RECOVERING STEAM-VOLATILE PARTIAL OXIDATION PRODUCTS OFCYCLOHEXANE FROM THE RESULTING MIXTURE BY DISTILLATION.